1. Field of the Invention This invention provides a method for using seismic reflection data to estimate residual receiver coordinates using reflection-data misalignments between traces observed in a CMP gather.
2. Discussion of the Prior Art
In the art of seismic surveying, a plurality of seismic receiver transducers, coupled to a signal-transmission cable, are distributed within an area of survey at prescribed intervals at or near the surface of the earth. Typically, the survey area may be divided into a (preferably) uniform grid, measuring, perhaps, 25 to 50 meters per side. The receiver transducers may preferably be located at the grid intersections. At designated stations or shot points, an energy source transducer fires a shot to generate a wavefield in the subsurface earth layers. The wavefield is reflected from the respective strata in the earth, ultimately returning to the surface. The receivers on the surface detect the reflected seismic waves, convert the mechanical seismic waves to electrical signals and transmit the resulting electrical signals through the cable to a recording device. The electrical signals are preferably recorded on a suitable recording medium for archival storage and later analysis.
Throughout the remainder of this disclosure, the term "transducer" is a collective term that includes both acoustic-signal receivers and acoustic energy sources. A transducer-specific qualifier will be used only when needed for clarity of context. If used, the transducer-specific terms will be limited to "source" or "receiver".
The recording device is usually a multi-channel data recorder. It is customary to couple the output from one or more receivers into each data channel. Several tens to several hundred data channels may be recorded from a single shot. The seismic signals flowing through the respective data channels are usually digitized and recorded on the recording medium in time-division multiplexed format as time scale recordings. The recording medium may be magnetic tape, a hard or floppy disk drive, a computer memory, optical disk or any other device now known or yet to be discovered.
The area to be surveyed is laid out with great care by a surveying crew, using methods suitable to the environment. At sea, various forms of radio ranging and/or satellite positioning are used. On land, transducer locations are referenced to geodetic bench marks, sometimes in combination with satellite positions. Despite the good intentions of the surveyors, the receiver emplacement engineers or the operators of the energy source may not be able to set up their instruments in the places originally designated. And even if they do, particularly at sea, ocean currents often cause some of the transducers to drift from their initial assigned locations.
Successful processing of the received seismic signals requires accurate knowledge of the relative configuration of the transducers as that configuration actually existed at the time of data recording. Various methods have been used in shallow-water marine operations for defining approximately-known receiver positions with respect to a known source position.
In U.S. Pat. No. 4,446,528 for a marine exploration system, a ship measures the water depth to the sensors mounted in a seismic bottom-cable as the ship passes over the cable. Next, the ship interrogates the sensors in the cable by means of sonar pulses along a slant range as the ship travels along a parallel, horizontally-offset path relative to the cable. The sensor locations are measured from recordings of the measured water depths combined with the slant-range travel times.
In another marine method, disclosed in U.S. Pat. No. 4,641,287, a series of seismic interrogation pulses are fired from different locations by an energy source. The distances from the shot locations to a sensor are determined by defining spherical surfaces upon which the sensor must be located. The intersection of several spherical surfaces derived from a plurality of shots determines the exact location of the sensor. Depth detectors may be used to eliminate one half of the possible locations for each shot.
The above two references are typical of known methods for use at sea but they are not of use for land operations. Another of the problems with the above references is that they assume that the interrogation signals are clean Dirac functions. That assumption is unrealistic because ambient noise severely degrades the interrogation signal, making it unreliable for precision use. Furthermore, special field operations and specialized field equipment are required, resulting in extra expense for the survey.
Copending U.S. patent application, Ser. No. 07/775,112, assigned to the assignee of this invention, teaches a statistical method for estimating the relative separation between a source and a receiver by use of the signature of the wavetrain that traveled along a substantially direct path between source and receiver. This process has the advantage of being useful on land as well as at sea and does not require special field equipment or operations. However, it does require additional special data-processing methods in the laboratory that are separate from conventional data-reduction routines.
In this disclosure, a method is taught in which the residual transducer coordinate errors can be estimated directly from information gathered during routine CMP data processing.